This invention relates to a method for determining exposure amount in photographic printing.
It is empirically known that the LATDs (Large Area Transmittance Density) of three primary colors of R (red), G (green) and B (blue) of a color original picture on a standard negative film are substantially constant. In the prior art photographic printing system, LATDs of three primary colors are photometrically measured on a negative film which is to be printed to control the amount of transmittance at a constant value for the contents of three primary colors. The prior art printing system can produce prints of high quality with well balanced colors in this manner so long as the color negative film is a standard type.
The controlling method by means of LATD, however, is not always effective for color original films where a specific color is dominant, and tends to produce defective prints with poor color balance. Dominance of a color(s) on a color original film is attributable to irregular distribution of colors on a object, influence from different light sources, regression of latent images in a layer sensitive to a specific color, etc. The photographic printing system generally has correction levels such as lowered-correction level and full-correction level by which colors of the original film with a dominant color(s) are effectively corrected. More particularly, the lowered-correction method gives a relatively lowered-exposure to the contents with relatively high LATD in each of the three primary colors of the original film and is quite effective as a means of color correction to prevent color failure caused by uneven distribution of colors on an object. The full-correction method, on the other hand, controls the exposure so as to make the result of integration of three colors neutral in printing and is quite effective as a means of color correction for the original film of which latent images are chronologically fading in the layer sensitive to a specific color or the original film affected by different light sources.
There have been proposed several improved methods for determining exposure amount which select correction levels. For example, Japanese Patent Laid-open No. 156624/1977, No. 156625/1977 and No. 145620/1978 disclosed a method for using photometric points selectively. The exposure amount is obtained by the method using photometric points included within a skin color region which is defined with an ellipsoid or an ellipse on three- or two-dimensional coordinates. Japanese Patent Laid-open No. 12330/1978 discloses a method which is capable of reproducing a predetermined desired color(s) (for example, colors of skin and of sky, green color, color of snow, etc.) on a sheet of photographic color paper. However, these papers do not disclose anything about the method for printing by changing color correction levels or by removing the contents of color failure although it can determine the exposure amount by using extraction of the color of a specific object. The method is detrimental in that it is extremely difficult to extract the color of a specific object precisely, and that it does not solve the problem caused when the specific color of the object does not exist or when it is extracted erroneously even if it exists. Japanese Patent Publication No. 29847/1984 discloses a method for determining exposure amount without the points which are colored relatively darkly. More particularly, points are used in color correction only if they satisfy at least two of the following three conditions: ##EQU1## wherein EQU D.sub.W =(D.sub.R +D.sub.G +D.sub.B)/3 EQU D.sub.NW =(D.sub.NR +D.sub.NG +D.sub.NB)/3
As this method does not use color coordinates, it is extremely difficult to change discriminating conditions by hues. Further, a problem exists in that the colors of which left term in the above expression (1) is very small or the colors of yellow, magenta and cyan are unconditionally selected. Under the condition of "D.sub.B -D.sub.G .ltoreq.D.sub.NB -D.sub.NG .+-.d1, D.sub.R -D.sub.G .ltoreq.D.sub.NR -D.sub.NG .+-.d1", the colors for the main of yellow, cyan and green are unconditionally selected. The method allows only a limited selection of photometric points and hence, is not very effective in removing the color failure components.
Japanese Patent Publication No. 15492/1981 describes a method of determining exposure amount which photometrically measures a frame for each predetermined segment, examines whether or not one of three primary colors is predominant, and if it is, the color is not used in determining exposure amount. For examining whether or not one of the primary colors is predominant, the ratios between two colors or B/G, G/R, R/B are compared with reference values. But the data on hues is not used as it does not use color coordinates. More particularly, according to this method, such information as related to different light sources (e.g. flourescent lamp, tungusten light, etc.) or to chronological changes on film (magenta) is excluded in determination of exposure amount. The method unavoidably prints film based on quite limited and sometimes erroneous data on photometric points. For instance, in the case of film exposed with a tungusten lamp, complementary colors mainly of the color of the light source are used as printing information and these colors are printed in colors similar to neutral, inconveniently enhancing the color of the tungusten lamp. Such error often occurs when not only different light sources are used but also when exposure is made with the light sources of low color temperature such as sunset or winter season. These films should be corrected in neutral gray color based mainly on the light source color points as well as the chronological color change points.
The method for determining exposure amount disclosed in Japanese Patent Laid-open No. 220760/1984 tries to improve discrimination of main hues in an under-exposed film by comparing the chromaticity of a photometric spot for the least dense portion with a marginal value and judging whether or not to use the value. It also attempts to reduce the dependency on the types of film. The paper, however, does not disclose anything about the improvement on the serious defects caused by films of different light sources or films with chronological color change. The method is also defective in that it takes much time in calculation as it requires to obtain distance between the center of each photometric spot and the vertex of a clor vector. Since this method uses the light-est point (or mask density) as the original point, it does not take into consideration the difference in sensitivity balance between red sensitive layer, green sensitive layer and blue sensitive layer and in gradient. The method therefore cannot distinguish main hues beyond a certain limit as it cannot precisely specify colors in terms of chromaticity based on the film type and the exposure amount at photographing time.
According to the above known methods, when the number of photometric points used in exposure amount determination is small, either photometric points of a reference original picture are used in the number to make up the number or no specific consideration is given at all. If the number of photometric points is small, the precision and stability of the exposure amount become low and the control of the exposure based upon them is improper. Even if the number of photometric points used is not very small, they are detrimental. For instance, images including blue sky or blue sea which need not be printed less densely are automatically printed lightly as those points are removed in advance. On the other hand, images including snow or cloudy sky which should be lightly printed are darkly printed as they are not removed in advance. As blue sky and blue sea tend to have hues similar to those of snow or cloudy sky, the density will inconveniently and widely change by the changes in film characteristics or in their saturation. The background of an object centering around the region which is marginal whether or not to be used in exposure control is liable to cause changes in density. Therefore, the prior art methods exclusively control either color balance or do not include such factors at all. There has long been felt a demand for a method for determining exposure amount which will not change the print of density by the types of images even if the number of photometric points is small.
There has been known a method for determining exposure amount which uses a correction amount predetermined by classification of features of color original pictures. Such methods are disclosed in Japanese Patent Laid-open No. 26568/1980, No. 26569/1980, No. 26570/1980 and No. 26571/1980. The methods examine the film consecutively by criteria such as the one with color failure, the one exposed with a fluorescent lamp, the one exposed with a tungsten lamp, an over-exposed one, an under-exposed one, a film with chronological changes, the one exposed at high color temperature, or the one exposed at low color temperature, allowing some overlaps between the criteria. But the methods have problems as the color correction factors are classified according to causes and complicate the methods inevitably. Although the methods theoretically distinguish the film exposed with a tungsten light from the film exposed at a low color temperature or the film exposed with an artificial light from a film with chronological changes, such classification per se is very difficult to make and there often are the case a film is not classified in either categories. The methods decribed above further do not show sufficient correction performance toward a red curtain under a fluorescent lamp, for example. Although it attempts to detect the color of the light source by examining the hues of skin color under various light sources and the same at the maximum density, these criteria in characteristic values are highly variable by the changes in film characteristics, color registration of three colors and color mixture when more than two color objects exist on one photometric point or various noises. As this method classifies films by the logical combination of characteristic values thus obtained, a film which almost but not quite satisfies a condition will be automatically excluded from an aimed category. Under such circumstances, there has long been awaited a method with an improved classification precision which can produce excellent prints with well balanced colors.